There have been many different types and kinds of battery chargers and battery charging systems. Such systems typically supply a constant high charge current to a battery for charging purposes, while monitoring the potential of the battery as it is being charged to protect the battery from being subjected inadvertently to an excessive charging current that could otherwise cause damage to the battery. For example, reference may be made to the following U.S. Pat. Nos.: 3,794,905; 3,886,427; 3,887,858; 4,163,934; 4,270,080; 4,342,954; 4,354,148; 4,388,852; 4,392,101; 4,503,378; 4,394,612; 4,609,860; 4,668,901; 4,710,694; 4,742,290; 4,767,977; and 4,746,852.
Typically, conventional nickel-cadmium batteries while being charged have a voltage-versus-time characteristic that rises slowly from an uncharged state to a fully charged state. Once such a battery has been fully charged, the charging current supplied to the battery must be switched rapidly from a high charge current rate to a slow or trickle charge current rate in order to prevent undesired and unwanted heat build-up in the battery that could cause irreparable damage to the battery and otherwise shorten its useful life.
In order to protect such batteries from being over charged, conventional battery charging systems have used peak detection circuits and the like for determining when the potential of a battery during the charging process has reached its maximum potential value to cause current switching to occur. While such prior known systems and methods have attempted to overcome such excessive heating problems, they have proven to be less than satisfactory for some applications. In this regard, because the voltage-versus-time characteristic curve of a typical battery under charge is not a smooth substantially linear waveform, certain deviations and fluctuations in the voltage characteristics of the battery have caused such prior known systems to detect false peak voltages and thus, in turn, have caused the charging of the battery to be prematurely terminated.
In an attempt to overcome this problem, many prior known battery charging systems have attempted to use microprocessors and analog to digital converters, to cause the switching to occur at a peak charging voltage only. In this regard, computerized techniques have been developed for determining when the voltage-versus-time characteristic curve of a battery has extended beyond a peak voltage value by examining whether the characteristic curve is exhibiting a negative slope.
While such prior known negative slope detecting circuits have tended to help overcome the aforementioned problems, such systems have proven to be less than satisfactory because certain voltage aberrations on the voltage-versus-time characteristic curve tend to produce false indications of a peak voltage, thus causing the microprocessor to evaluate slope characteristics improperly.
The U.S. Pat. No. 4,806,840 discloses an analog to digital converter and microprocessor with a special algorithm for the purpose of attempting to establish with certainty that the voltage-versus-time characteristic has extended well into the negative slope portion of the curve before terminating the charging current. In this regard, the microprocessor firmware required that a negative slope indication be determined on two successive time intervals before enabling the battery charging current to be switched. However, in order to establish when to commence calculating the two successive time intervals, the system firmware still requires the detection of a peak voltage and is still subject to false peak voltage detections due to low frequency aberrations in the battery voltage potential. More particularly, if a battery is improperly secured between the jacks or terminals in a charging station, or the charging equipment is jarred while charging the battery, aberrations can occur and can produce false peak voltage indications.
Therefore, it would be highly desirable to have a new and improved battery charging system which overcomes more satisfactorily the foregoing-mentioned problems associated with signal aberrations occurring during a charging operation, Such a battery charging system, should enable a precise and accurate determination of a peak voltage condition, so that the charging current may be properly switched from a hard charge mode of operation. The charging system should also be relatively simple to operate and maintain, and should be relatively inexpensive to manufacture.